Formats for storing Hi-C pairs

.pairs

.pairs is a simple tabular format for storing DNA contacts detected in a Hi-C experiment. The detailed .pairs specification is defined by the 4DN Consortium.

The body of a .pairs contains a table with a variable number of fields separated by a “\t” character (a horizontal tab). The .pairs specification fixes the content and the order of the first seven columns:

index name description
1 read_id the ID of the read as defined in fastq files
2 chrom1 the chromosome of the alignment on side 1
3 pos1 the 1-based genomic position of the outer-most (5’) mapped bp on side 1
4 chrom2 the chromosome of the alignment on side 2
5 pos2 the 1-based genomic position of the outer-most (5’) mapped bp on side 2
6 strand1 the strand of the alignment on side 1
7 strand2 the strand of the alignment on side 2

A .pairs file starts with a header, an arbitrary number of lines starting with a “#” character. By convention, the header lines have a format of “#field_name: field_value”. The .pairs specification mandates a few standard header lines (e.g., column names, chromosome order, sorting order, etc), all of which are automatically filled in by pairtools.

The entries of a .pairs file can be flipped and sorted. “Flipping” means that the sides 1 and 2 do not correspond to side1 and side2 in sequencing data. Instead, side1 is defined as the side with the alignment with a lower sorting index (using the lexographic order for chromosome names, followed by the numeric order for positions and the lexicographic order for pair types). This particular order of “flipping” is defined as “upper-triangular flipping”, or “triu-flipping”. Finally, pairs are typically block-sorted: i.e. first lexicographically by chrom1 and chrom2, then numerically by pos1 and pos2.

Pairtools’ flavor of .pairs

.pairs files produced by pairtools extend .pairs format in a few ways.

  1. pairtools store null/ambiguous/chimeric alignments as chrom=’!’, pos=0, strand=’-‘.
  2. pairtools store the header of the source .sam files in the ‘#samheader:’ fields of the pairs header. When multiple .pairs files are merged, the respective ‘#samheader:’ fields are checked for consistency and merged.
  3. Each pairtool applied to .pairs leaves a record in the ‘#samheader’ fields (using a @PG sam tag), thus preserving the full history of data processing.
  4. pairtools append an extra column describing the type of a Hi-C pair:
index name description
8 pair_type the type of a Hi-C pair

Pair types

pairtools use a simple two-character notation to define all possible pair types, according to the quality of alignment of the two sides. The type of a pair can be defined unambiguously using the table below. To use this table, identify which side has an alignment of a “poorer” quality (unmapped < multimapped < unique alignment) and which side has a “better” alignment and find the corresponding row in the table.

. . . Less informative alignment More informative alignment .
Pair type Code >2 alignments Mapped Unique Mapped Unique Sidedness
walk-walk WW 0 [1]
null NN     0
corrupt XX     0 [2]_
null-multi NM   0
null-rescued NR   1 [3]
null-unique NU   1
multi-multi MM 0
multi-rescued MR 1 [3]
multi-unique MU 1
rescued-unique RU 2 [3]
unique-rescued UR 2 [3]
unique-unique UU 2
duplicate DD 2 [4]_
[1]“walks”, or, C-walks are Hi-C molecules formed via multiple ligation events which cannot be reported as a single pair.
[2]“corrupt” pairs are those with technical issues - e.g. missing a FASTQ sequence/SAM entry from one side of the molecule.
[2]“rescued” pairs have two non-overlapping alignments on one of the sides (referred below as the chimeric side/read), but the inner (3’-) one extends the only alignment on the other side (referred as the non-chimeric side/read). Such pairs form when one of the two ligated DNA fragments is shorter than the read length. In this case, one of the reads contains this short fragment entirely, together with the ligation junction and a chunk of the other DNA fragment (thus, this read ends up having two non-overlapping alignments). Following the procedure introduced in HiC-Pro and Juicer, pairtools parse rescues such Hi-C molecules, reports the position of the 5’ alignment on the chimeric side, and tags them as “NU”, “MU”, “UR” or “RU” pair type, depending on the type of the 5’ alignment on the chimeric side. Such molecules can and should be used in downstream analysis. Read more on the rescue procedure in the section on parsing.
[3](1, 2, 3, 4) pairtools dedup detects molecules that could be formed via PCR duplication and tags them as “DD” pair type. These pairs should be excluded from downstream analyses.

.pairsam

pairtools also define .pairsam, a valid extension of the .pairs format. On top of the pairtools’ flavor of .pairs, .pairsam format adds two extra columns containing the alignments from which the Hi-C pair was extracted:

index name description
9 sam1 the sam alignment(s) on side 1; separate supplemental alignments by NEXT_SAM
10 sam2 the sam alignment(s) on side 2; separate supplemental alignments by NEXT_SAM

Note that, normally, the fields of a sam alignment are separated by a horizontal tab character (\t), which we already use to separate .pairs columns. To avoid confusion, we replace the tab character in sam entries stored in sam1 and sam2 columns with a UNIT SEPARATOR character (\031).

Finally, sam1 and sam2 can store multiple .sam alignments, separated by a string ‘\031NEXT_SAM\031’